In general, a separator for separating the active material for the positive electrode and the active material for the negative electrode is used in alkaline primary batteries. Various properties such as those shown in the following are required for the separator.    1. Internal short circuit between the active material for the positive electrode and the active material for the negative electrode is prevented.    2. The absorption of the electrolyte is great, the ion conductivity is excellent, and the electric resistance is small so that the sufficient electromotive reaction can proceed.    3. When the separator is disposed at the inside of the battery, the volume occupied by the separator is small so that the amounts of the active material for the positive electrode and the active material for the negative electrode can be increased (the period of time of the use of the battery can be increased).    4. When the separator is disposed at the inside of the battery, the separator itself is not buckled by the impact of vibration and dropping while the battery is delivered or carried, and the short circuit does not take place.
To obtain the above properties, a separator for batteries in which a polyvinyl alcohol-based fiber exhibiting excellent chemical resistance, hydrophilic property and mechanical properties is used, and a cellulose-based fiber is used in combination to enhance the property of absorbing the electrolyte, is proposed (Patent Reference 1).
However, a problem arises in that the short circuit takes place due to so-called dendrite which is needle shaped zinc oxide separated by the action of the aluminum component added to zinc constituting the negative electrode to suppress generation of hydrogen in the electrolyte, and an abnormal decrease in the voltage occurs. As the result, the life of the battery is decreased.
Another problem arises in that the mixture of manganese dioxide and graphite used as the material of the positive electrode in the conventional alkaline manganese batteries causes oxidative degradation of the cellulose fiber present on the face of contact between the separator for alkaline manganese batteries and the positive electrode mix during storage at high temperatures, and the capacity of the positive electrode is decreased. As the result, the properties of the battery deteriorate.
On the other hand, further improvements in the properties of the battery are required for alkaline batteries due to the recent rapid growth in the use and the improvements in the properties of various digital instruments. The electric power applied to the instruments using a battery is gradually increasing, and a battery exhibiting an excellent discharging property under a great load is desired. An alkaline battery exhibiting an improved discharging property under a great load by using a mixture obtained by adding nickel oxyhydroxide to manganese dioxide as the active material for the positive electrode is practically used. However, since nickel oxyhydroxide has a stronger oxidizing ability than manganese oxide has, a problem arises in that the cellulose fiber in the above separator for alkaline batteries present at the face contacting the positive electrode mix is oxidatively degraded extremely rapidly, and the discharging property under a great load deteriorates.
To prevent the internal short circuit between the active material for the positive electrode and the active material for the negative electrode, it is proposed that a polyvinyl alcohol-based fiber and a cellulose fiber are treated by beating to form an alkali-resistant fiber, and a separator for batteries having a double layer structure is prepared with a combination of a dense layer having a great density of the fiber and a layer for holding fluids having a small density of the fiber (a coarse layer) (Patent Reference 2). However, it is difficult that the excellent property of absorbing fluids and the function of preventing the internal short circuit are simultaneously exhibited. When the relative amount of the dense layer is increased to prevent the short circuit due to dendrite, a problem arises in that the amount of the absorbed fluid decreases, and the internal short circuit takes place due to the insufficient amount of the absorbed fluid. As the result, the life of the battery decreases. When a highly beaten cellulose fiber is used for the dense layer, the buckling of the separator itself takes place by the impact of vibration and dropping while the battery is delivered or carried since the buckling resistance of the separator itself decreases, and the internal short circuit takes place.
There is still another problem in that the cellulose fiber present at the face of the separator contacting the manganese dioxide used as the active material for the positive electrode is oxidatively degraded, and the life of the battery decreases. Naturally, the cellulose fiber is more rapidly oxidatively degraded in a high performance battery (a battery exhibiting the excellent discharging property under a great load) using a substance for the positive electrode containing nickel oxyhydroxide, and the discharging property under great loads deteriorates.
On the other hand, a separator using a cellophane film in combination with paper or a nonwoven fabric using a combination of an alkali-resistant fiber and a cellulose fiber is adopted to prevent the internal short circuit. However, the above separator exhibits an inferior property of absorbing fluids, and it is necessary that the paper base material is used in a great amount to surely obtain the desired amount of the absorbed fluid. Therefore, the volume occupied with the separator increases in the battery, and the amounts of the active material for the positive electrode and the active material for the negative electrode are restricted. Due to the use of cellophane film and the increased amount of the paper base material, the distance between the electrodes increases, and the internal resistance increases. As the result, the discharging property of a great capacity cannot be obtained.
To enhance the property of absorbing fluids of the paper base material comprising the alkali-resistant fiber, separators for batteries prepared by impregnation and coating with 0.5 to 1.0 g/m2 of a highly hygroscopic macromolecular compound of the crosslinking type are proposed (Patent References 3 to 5). However, since these separators cannot suppress the growth of dendrite, a problem arises in that the effective prevention of the short circuit due to dendrite is difficult, and the internal short circuit takes place.
An alkaline battery using a negative electrode mix made of an electrolyte in the gel form which contains a compound having silicon in a specific amount is proposed (Patent Reference 6). Although the above battery satisfies the requirement of the discharging property (in the test of discharge after the battery is kept at 60° C. for 1 month while the battery is intermittently discharged at 3.9Ω for 5 minutes every day), a problem arises in that buckling of the separator itself takes place after being dropped, and the internal short circuit takes place. Another problem arises in that the cellulose fiber at the face contacting the positive electrode mix containing manganese dioxide and graphite as the main components which is used in conventional alkaline batteries or the positive electrode mix using a combination of manganese dioxide and nickel oxyhydroxide exhibiting strong oxidizing ability (exhibiting an excellent discharging property under a great load) which is used in high performance batteries is oxidatively degraded, and the gap increases. As the result, the properties of the battery deteriorate due to the internal short circuit and the decrease in the capacity of the positive electrode.
Patent Reference 1: Japanese Patent Application Laid-Open No. Heisei 6 (1994)-163024
Patent Reference 2: Japanese Patent Application Laid-Open No. Heisei 10 (1998)-92411
Patent Reference 3. Japanese Patent Application Laid-Open No. Showa 57 (1982)-105957
Patent Reference 4: Japanese Patent Application Laid-Open No. Showa 57 (1982)-105958
Patent Reference 5: Japanese Patent Application Laid-Open No. Heisei 2 (1990)-078150
Patent Reference 6: Japanese Patent Application Laid-Open No. Heisei 9 (1997)-035720